This invention relates to digital data communication, and more particularly, to a method and apparatus for multiplexing or concentrating multiple digital signals of substantially arbitrary word formats and speed into a single digital data stream using time division multiplexing techniques suitable for high speed trunk-type transmission, as between ground stations through a satellite.
Digital data communication systems have used word-by-word Time Division Multiplexing (TDM) formats having fixed-sized word slots at fixed intervals within a frame. Such systems are not able to accommodate a frequently-changing mix of signals characterized by varying sample rates and word sizes. The channel utilization factor is not maximum, and resynchronization may be necessary. Such a system is extremely difficult to control under circumstances where it is desireable to make maximum use of the available channel. It is also difficult to maintain precise relative timing information among the signals which are output from a demultiplexer.
What is needed is a technique for generating time division multiplexed information from digital data of multiple formats in a manner which maximizes channel utilization.
The following patents were uncovered in a search of the records of the United States Patent and Trademark Office in relation to the subject invention.
U.S. Pat. No. 3,982,074 to Clark describes an automatic channel assignment circuit in which there is disclosed an asynchronous multiplexer/demultiplexer system for multiplexing multiple signals having variable bit rates into a single transmitted data stream having a predetermined fixed data format consisting of subframes.
U.S. Pat. No. 3,982,077, to Clark et al., describes an asynchronous multiplexer and demultiplexer combination in which is disclosed an asynchronous multiplexer and demultiplexer system for multiplexing multiple digital signals having variable bit rates into a single transmitted data stream, having a predetermined fixed data format.
The Clark patents describe a system where the bits are spread uniformly throughout a data stream to minimize the buffering requirement. "Stuff" bits are used to take into account areas of the data stream where no real data is used. As a consequence, data channel utilization is diminished by the substitution of "stuff" bits for useful data.
In neither of the above patents is there a provision for variable data formats. Therein equal spacing is required between both transmitted and received time slots, and the data rates cannot change over time. In particular, the Clark devices would not work on a type of system requiring dynamic bit streams.
U.S. Pat. No. 4,258,434, to Glowinski et al., describes a bit-by-bit time-division digital switching network for binary data streams. Therein, bit-by-bit time division digital switching on an internal bus multiplexes a plurality of isochronous bit-serial data channels having rates which are different but submultiples of a given maximum rate. The technique associates an address word with bit of an input channel, which is an undesirable ratio of overhead to information in a communication channel.
U.S. Pat. No. 3,987,251, to Texier et al., and U.S. Pat. No. 3,988,544, to Texier et al., describe time division switching networks for switching digital data, where the rates, the number of active channels and the formats of each channel are static. It is required that the rates be multiples of one another in order to function as intended. In the multiple-format system, small format words are used to form large format words. Hence, it appears that the combination cannot be arbitrary.
U.S. Pat. No. 3,749,842, to Poretti, describes a time slot allocation network for a multiplexed telecommunication system and is cited for background information.
U.S. Pat. No. 3,752,933, to Cohen et al., describes a bit regeneration technique for time division multiplexers. It is cited as background information.
U.S. Pat. No. 3,790,715, to Inose et al., describes a digital transmission terminal for voice and low speed data. It employs time division multiplexed transmission channels which are frame based. It is cited primarily for background information.
U.S. Pat. No. 4,429,383, to Finck et al., describes a method for seizing time slots of a time-division multiplexed system having dynamic multipliers. This system also relies on fixed time slots and is relevant primarily as background information.
U.S. Pat. No. 4,471,480, to Haussmann et al., describes a programmable controller for a time-division multiplexed digital multiplexer-demultiplexer combination. A common controller is used to control synchronous input and output of data signals or asynchronous input and output of data signals, wherein the data signals are distributed throughout a fixed data format in time slots using automatic channel assignment. This patent is cited primarily for background information.
U.S. Pat. No. 4,631,720, to Koeck, describes a service integrated transmission system. This system is a frame-based system wherein the frequency of the time multiplexed frame is selected to be equal to the smallest information rate of the signals to be transmitted. This invention therefore presupposes knowledge of the expected data rates. This patent is included primarily for background information.
U.S. Pat. No. 4,646,290, to Hills, describes a data transmission method and apparatus which is intended for use with an FM subcarrier data transmission system wherein a multiple-frame matrix is constructed using an arrangement of data messages of varying bit rates in real time. This patent is included primarily for background information.
U.S. Pat. No. 4,658,152, to Walters, describes an adaptive rate multiplexer-demultiplexer. The system described is a frame-based system involving reassignment of subchannels.
What is needed is a multiplexer system for a data communication system which is capable of handling substantially arbitrary groupings of data of diverse transmission speeds, so that information from multiple sources can be combined, conveyed and then separated with a minimum of loss in transmission efficiency and at extremely high speeds.